Wireless communication devices typically include a frequency synthesizer, which is often a critical component of many techniques used to modulate data onto an outgoing wireless signal. Conventional modulation techniques, such as Gaussian Frequency Shift Keyed (GFSK) modulation, are typically implemented using either I-Q up-conversion or by direct open-loop modulation of a voltage-controlled oscillator (VCO). The up-conversion approach is often susceptible to an undesired image signal and is typically costly in terms of analog area and power. The open-loop modulation approach is often prone to frequency drift in the voltage-controlled oscillator and to gain errors.